Effective Range of Viewing Instruments

The distance at which an observer can just perform his particular visual observation task is perhaps the most practical metric of image quality with regard to viewing instruments. The direct relation between the distance of a target and the scaling of its image upon the retina suggests a simple theoretical approach: is it possible to express image quality in some sort of effective retinal "pixel-size", such as visual acuity, but then in a more generalized form, including the effects of luminance level, contrast, and noise? In this paper experiments will be described in order to determine the effective range first of a typical image intensifier system, and second for a thermal viewing system. The results will be discussed in the light of the above question.